The present invention relates generally to electrical circuits and more particularly to a controlling circuit operable generally to maintain the effective value of an alternating current voltage applied to a load near a desired value.
In the past, many voltage regulating circuits were known having direct current regulation accomplished by devices, such as zener diodes or the like, where voltage is substantially independent of current flow once the zener voltage has been exceeded. Regulation of peak alternating current voltages was similarly accomplished by utilizing means well known to the art for "clipping" peak values thereof; however, such clipping to regulate peak voltage does not simultaneously regulate the effective voltage. This follows since, the greater the degree of clipping, the more nearly the applied voltage waveform approximates a square wave and the greater the effective or RMS value of that voltage becomes.
Systems or circuits for regulating the effective or RMS value of an applied voltage have also been proposed. Some of these circuits employed optical feedback paths and also frequently employed silicon controlled rectifiers or other similar devices. In this manner, the relative phase at which the silicon controlled rectifier was triggered determined the effective value of the voltage applied to the load, and the optical feedback path responsive to a lamp or the like in parallel with the load controlled this phase angle thereby to maintain the effective voltage at or near a desired level. Such controlling circuits typically employed relatively expensive transformers and numerous circuit elements resulting in a relatively expensive control system. Further, such controlling circuits frequently supplied only the phase angle determined portions of successive current excursions of like polarity to the load and thus constituted a type of half-wave rectification prior to the load. For the foregoing as well as other reasons, such known controlling circuits have been unsuited to low cost applications, such as regulating the effective voltage applied to a projection lamp in photo projection equipment, such as slide projectors, overhead projectors, and movie projectors or the like.